I'm guessing the compressor is about to go. Let me list my symptoms:
Unit usually runs ok. Will tend to shut down despite the desired temp not being met. After the typical 3 or 4 minute period it starts back up and usually continues cooling. This happens a fair bit every day under heavy use (once an hour maybe? maybe twice).
Every so often (four times in the last two months), the fan will come on but the compressor will not turn on. It will shut down after the fan runs without the compressor after about 20 or 30 seconds.
After some time it will usually start back up again and work ok. Keep in mind I've been shutting the unit down when this happens and killing the breaker. Not sure why, just seemed the thing to do.
Capacitor seems to be ok I checked the voltage on turn on (when the compressor would fire up) and it was fine. Although I have not been on the roof checking it when the compressor wouldn't fire. These things usually work or don't work though so I've been ruling out a bad cap. Especially since it does work most of the time.
Figured the compressor is on its last legs. Sound about right? We use the heck out of that unit and in high temps. Its probably seen more use in the last 5 years than most units see in 20.